1. Field of the Invention
The present invention relates to an additive for drilling fluid used as a sealing agent to reduce seepage loss and to reduce fluid loss in subterranean wellbores.
2. Prior Art
Drilling fluids or drilling “muds” serve a number of functions during the drilling of an oil or gas well. The fluids lubricate and cool the drill bit and carry drill cuttings to the surface. In addition, the drilling fluid can have major interactions with the wellbore, either helping to stabilize the hole or creating problems with the wellbore.
A myriad of different fluids have been developed in the past to fulfill the functions required when drilling. The two basic types are water-based and oil-based. The water-based fluids can be further divided into several categories, two of which are bentonite containing fluids and polymer systems.
Drilling fluids interact with the formations being drilled. If the formations are at all porous or fractured, the drilling fluid can migrate into them by capillary action and slow percolation (seepage loss). While there are several ways that the fluid interacts with the formation, one key to preventing down hole problems is physically plugging the pores or fractures to prevent or reduce fluid migration.
The material used to plug the openings needs to be sized appropriately for the size of opening to be plugged. Fluid migration of drilling fluids into formations falls into three general categories: fluid loss, seepage loss, and lost circulation. Fluid loss is the migration of the liquid phase of the drilling fluid into the formation. The very fine particles in drilling fluids are designed to form a filter cake on the surface of the formation which prevents solid migration into the formation, but some liquid may still penetrate, if more slowly.
If the formations are fractured or have large holes or vugs, the filter cake may not be sufficient to prevent liquid and/or entire drilling fluid from entering the formation. If so much drilling fluid is being lost that little or no drilling fluid is being returned to the earth's surface, the situation is described as “lost circulation”. In this circumstance, very large particles may be required to plug the formation.
When the formations being drilled are very porous or contain small fractures, more fluid may be lost than would be expected with ordinary fluid loss, but less than with lost circulation. This situation is termed “seepage loss”.
The present invention is directed to an additive for drilling fluid composed of bituminous coal having a selected particle size distribution and a median particle size which is added to drilling fluid at a selected rate.
Products Used for Sealing:
A variety of products have been used to provide sealing in drilling fluids. One of the most common is calcium carbonate (CaCO3) because it is acid soluble. Acid solubility is important when drilling a producing formation because the CaCO3 can be removed with acid once production is desired. A number of companies supply CaCO3 in a variety of particle sizes. A few examples are CalCarb-Titan 200 and CalCarb 80 from Geo Drilling Fluids, Inc. (see http://www.geodf.com/store/geodf_products/4.pdf), Glo Carb in extra fine, fine, medium, and coarse from Global Drilling Fluids & Chemicals LTD (see http://globaldrillchem.com/products/index/clacium-carbonate_3953.html), and Calcium Carbonate in fine, medium, and coarse from Weatherford (see www.weatherford.com/weatherford/groups/web/documets/weatherfordcorp/wft126333.pdf).
Other sealing products fall in the category of asphaltites. One of these mined products is found in the Uintah Basin of Utah and has the mineral name uintaite. The product can be treated to make it more dispersible in water-based fluids. The untreated uintaite is sold commercially as Gilsonite®, GSX-601®, and Gilsonite GM-P®. The treated uintaite is sold commercially as Treated Gilsonite®, GSX-509®, and Gilsonite DG-P®. Similar natural asphalts are mined in Columbia and sold commercially as Gilsocol GP® (untreated) and Gilsocol DG® (treated).
Graphite is another product used in drilling fluids. It is primarily used as a lubricant, but is also used to reduce fluid loss. One example is SX-Plus from Sun Drilling (see http//www.sundrilling.com/SX-Plus_drilling_mud_fluid_additive.php). Graphite is also available from other companies such as M-I Swaco (G-Seal) (see http://www.slb.com/˜/media/Files/miswaco/product_sheets/G-SEAL.pdf). Requirements for Sealing:
Several factors are involved in achieving sealing during the drilling operation. Among the most critical factors are the total volume of solids and the size distribution of the particles compared to the size of the formation pores or fractures to be sealed. Other factors include the nature of the drilling fluid, the shape of the particles, and the amount and types of polymers in the system.
Generally speaking, the more particles present in the drilling fluid, the easier it is to obtain sealing. Thus, high-solid fluids such as bentonite-containing muds or weighted muds will seal more easily than low-solid fluids such as polymer systems or unweighted muds. Systems such as an X-C polymer fluid with no drill solids do not seal well at all. One rule of thumb proposed by Abrams stated that the concentration of bridging solids should be at least 5% by volume. See Abrams, A., “Mud Design to Minimize Rock Impairment Due to Particle Invasion”, JPT (May 1977) 586. Under ideal conditions, Dick et al. found that sealing would occur at lower volumes. See Dick, M. A., Heinz, T. J., Svoboda, C. F., and Aston, M., “Optimizing the Selection of Bridging Particles for Reservoir Drilling Fluids”, SPE 58793, 2000 SPE International Symposium on Formation Damage, Lafayette, La., 23-24 Feb. 2000.
Abrams also proposed that the median particle size of the bridging particles should be equal to or slightly greater than ⅓ the median pore size of the formation. Thus, to seal porous sandstones with large pore sizes, larger bridging particles would be required than for less porous rocks with much smaller pore sizes. The size D90 is defined as the particle size at which 90% of the particles are smaller than that size. Another sizing theory proposed by Hands et al. suggested that the D90 of the bridging particles should be equal to or less than the pore size of the rock. See Hands, N. et al., “Drill-In Fluid Reduces Formation Damage, Increases Production Rates”, Oil and Gas Journal (July 1998) 13.
As mentioned earlier, a variety of materials have been used to control fluid losses. For example, in U.S. Pat. No. 5,004,553, House et al. describes the use of ground oat hulls alone or in combination with ground corn cobs, hydrophobic organophilic water wettable cotton, ground citrus pulp, and ground cotton burrs. The oat hulls in the House patent were specified to be 95% less than 590μ and 90% greater than 75μ. The other materials used in combination with the oat hulls were 75% less than about 250μ. These particle size distributions would be significantly larger than any of the mud grade products described in the present invention. It is important to note that the House patent does not mention the use of coal of any variety.
Pomerleau (U.S. Patent Publication No. 2010/0230164) addressed lost circulation, seepage loss, and fluid loss control. The patent describes the use of ground pumice, barium, dolomite, anthracite or a combination of these materials in drilling fluids. The patent covers a very broad range of particle sizes (between 100 and 4,000 microns) and an extremely wide range of concentrations (0.01 to 300 ppb). While the patent does suggest anthracite as one possible additive, it does not mention bituminous coal which is softer than anthracite and results in a very different particle size distribution when processed through a mill. Further, as will be set forth in detail, the products in the present invention have a much more restricted particle size range and concentration range.
Messenger (U.S. Pat. No. 3,788,406) describes the use of coal in a high concentration slurry (a “pill”) to control lost circulation. Generally, lost circulation requires much larger particles than would be used to control seepage loss. No specific type of coal is mentioned and the particle size range (75 to 4760μ) is much broader than the products described in the present invention. Such a pill would not be suitable for the present seepage loss or a fluid loss situation.
Cardwell et al. (U.S. Pat. No. 2,650,195) describes a method of using resin to coat particles to prevent loss of fluid into thief formations. This again is a lost circulation rather than seepage loss situation and as such, the particles discussed are much larger than the products in the present invention. Coal, coke, cinders, and nut shells are mentioned as possible filler materials, but the type of coal is not specified.
Notwithstanding the foregoing, it would be desirable to provide an additive for drilling fluid used as a sealing agent to reduce seepage loss and fluid loss with a readily available material with a selected particle size distribution and a median particle size having improved efficiency.